It is adding fixed style button on the left of the admin area and allows to hide/display side panel.

This is useful for smaller screens, when main content table float below side area and looks not very good.

Also, it remembers state of the toggler inside cookie and on the next page visit it applies this setting immediately.

Advantages of writing code like this component, which is based on core events:

* Loose coupling of components between each other (event listener and and/or event fires can be absent and nothing breaks)

* There can be several listeners for one event, this is especially useful for plugins system, each new plugin can listen to existing events and can optionally fire new events, not existed before that plugin arrived.

* Pieces of code starting to look like easier and smaller, no need to think how to inject some piece of code into specific engine processing stage.

* Writing events listeners into separate file (optionally) or inside any other place of code (class or function)